1. Field of the Invention
This invention relates to a thermal head for use in a heat sensitive type printer or heat transfer type printer. More particularly, this invention concerns an insulating heat conductive layer disposed just below the protection layer.
2. Description of the Prior Art
The conventional thermal head has a cross sectional structure, for example, as shown in FIG. 2 prepared by using a partially glazed substrate in which a glazed layer 2 is partially formed on the surface of an insulating substrate 1 such as made of alumina and by laminating thereover an undercoat layer 3' such as made of a Ta.sub.2 O.sub.5 film, a heat generating resistor layer 4 such as made of a Ta.sub.2 N or Ta-W-N film, an electric power supply conductor layer 5 such as made of an Al film and a protection layer 6 such as made of a dual layer film composed of a lower SiO.sub.2 layer film and an upper Ta.sub.2 O.sub.5 layer film successively. A part of the heat generating portion 7 generates heat to provide heat energy for developing color on heat sensitive paper (not illustrated) or the like in contact therewith.
The conventional thermal head involves the following problems. FIG. 3(b) shows the result of the step-stress test (SST) for the conventional thermal head as described above. In this case, SST is a sort of acceleration tests for evaluating the heat-resistant stability of the thermal head, in which an appropriate pulse voltage is applied for a predetermined length of time to the heat generating resistor body to measure the variation relative to the initial resistance value. The variation coefficient for the resistance value in each of the steps is plotted while gradually increasing the application voltage till the heat generating resistor body is burnt to be disconnected. If the variation coefficient for the resistance value increases in excess of 20%, the heat generating resistor body is judged to be destructed.
Now, FIG. 3(b) shows the result of SST for the conventional thermal head having a plurality of heat generating portions (dots). As can be seen from the figure, destruction already appears at a relatively low electric power applied and there is a drawback that the destruction voltage varies depending on the dots. Generation of dots destructed at such a low electric applied power significantly degrades the reliability of the thermal head to result in a significant problem.